// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// This module provides a way to monitor a file or directory for changes.

#ifndef BASE_FILES_FILE_PATH_WATCHER_H_
#define BASE_FILES_FILE_PATH_WATCHER_H_

#include "base/base_export.h"
#include "base/callback.h"
#include "base/files/file_path.h"
#include "base/macros.h"
#include "base/memory/ref_counted.h"
#include "base/single_thread_task_runner.h"

namespace base {

// This class lets you register interest in changes on a FilePath.
// The callback will get called whenever the file or directory referenced by the
// FilePath is changed, including created or deleted. Due to limitations in the
// underlying OS APIs, FilePathWatcher has slightly different semantics on OS X
// than on Windows or Linux. FilePathWatcher on Linux and Windows will detect
// modifications to files in a watched directory. FilePathWatcher on Mac will
// detect the creation and deletion of files in a watched directory, but will
// not detect modifications to those files. See file_path_watcher_kqueue.cc for
// details.
class BASE_EXPORT FilePathWatcher {
public:
    // Callback type for Watch(). |path| points to the file that was updated,
    // and |error| is true if the platform specific code detected an error. In
    // that case, the callback won't be invoked again.
    typedef base::Callback<void(const FilePath& path, bool error)> Callback;

    // Used internally to encapsulate different members on different platforms.
    class PlatformDelegate : public base::RefCountedThreadSafe<PlatformDelegate> {
    public:
        PlatformDelegate();

        // Start watching for the given |path| and notify |delegate| about changes.
        virtual bool Watch(const FilePath& path,
            bool recursive,
            const Callback& callback) WARN_UNUSED_RESULT = 0;

        // Stop watching. This is called from FilePathWatcher's dtor in order to
        // allow to shut down properly while the object is still alive.
        // It can be called from any thread.
        virtual void Cancel() = 0;

    protected:
        friend class base::RefCountedThreadSafe<PlatformDelegate>;
        friend class FilePathWatcher;

        virtual ~PlatformDelegate();

        // Stop watching. This is only called on the thread of the appropriate
        // message loop. Since it can also be called more than once, it should
        // check |is_cancelled()| to avoid duplicate work.
        virtual void CancelOnMessageLoopThread() = 0;

        scoped_refptr<base::SingleThreadTaskRunner> task_runner() const
        {
            return task_runner_;
        }

        void set_task_runner(scoped_refptr<base::SingleThreadTaskRunner> runner)
        {
            task_runner_ = std::move(runner);
        }

        // Must be called before the PlatformDelegate is deleted.
        void set_cancelled()
        {
            cancelled_ = true;
        }

        bool is_cancelled() const
        {
            return cancelled_;
        }

    private:
        scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
        bool cancelled_;
    };

    FilePathWatcher();
    virtual ~FilePathWatcher();

    // A callback that always cleans up the PlatformDelegate, either when executed
    // or when deleted without having been executed at all, as can happen during
    // shutdown.
    static void CancelWatch(const scoped_refptr<PlatformDelegate>& delegate);

    // Returns true if the platform and OS version support recursive watches.
    static bool RecursiveWatchAvailable();

    // Invokes |callback| whenever updates to |path| are detected. This should be
    // called at most once, and from a MessageLoop of TYPE_IO. Set |recursive| to
    // true, to watch |path| and its children. The callback will be invoked on
    // the same loop. Returns true on success.
    //
    // Recursive watch is not supported on all platforms and file systems.
    // Watch() will return false in the case of failure.
    bool Watch(const FilePath& path, bool recursive, const Callback& callback);

private:
    scoped_refptr<PlatformDelegate> impl_;

    DISALLOW_COPY_AND_ASSIGN(FilePathWatcher);
};

} // namespace base

#endif // BASE_FILES_FILE_PATH_WATCHER_H_
